Many modern aircraft employ gas turbine engines for propulsion. Such engines include a fan, compressor, combustor and turbine provided in serial fashion, forming an engine core, and arranged along a central longitudinal axis. Air enters the engine through the fan and is pressurized in the compressor. This pressurized air is mixed with fuel in the combustor. The fuel-air mixture is then ignited, generating hot combustion gases that flow downstream to the turbine. The turbine is driven by the exhaust gases and mechanically powers the compressor and fan via an internal shaft. Energy from the combustion gases not used by the turbine is discharged through an exhaust nozzle, producing thrust to power the aircraft.
Turbofan engines contain an engine core and fan surrounded by a fan cowl, forming part of the nacelle. The nacelle is a housing that contains the engine. The fan is positioned forward of the engine core and within the fan cowl. The engine core is surrounded by an engine core cowl and the area between the fan cowl and the engine core cowl is functionally defined as the fan duct. This fan duct is substantially annular in shape to accommodate the airflow from the fan and around the engine core cowl. The airflow through the fan duct, known as bypass air, travels the length of the fan duct and exits at the aft end of the fan duct at a fan nozzle. The fan nozzle is comprised of an engine core cowl disposed within a fan cowl and is located at the aft portion of the fan duct.
In addition to thrust generated by combustion gasses, the fan of turbofan jet turbine engines also produces thrust by accelerating and discharging ambient air through the fan exhaust nozzle. The fan includes a plurality of blades mounted to a central hub. Each blade includes a tip, distal to the central hub, in close proximity to a rub strip along the nacelle interior. The rub strip is a section of the nacelle interior closest to the tip. In a variable-pitch design, the angle of the blades may be adjusted relative to the rub strip to provide multiple propulsion modes. Individual blades are inserted into blade receivers that can adjust the blade angle. As the blade angle changes, the tip rotates relative to the rub strip.
To maintain a desired amount of clearance between the blade and the rub strip while allowing a variable-pitch design, both the tip and the rub strip may be spherically shaped. However, as a rub strip may have a leading edge with a smaller inner diameter than that of a rub strip center section, it may be impossible to insert the blade into the blade receiver axially along the central longitudinal axis, as the tip will not clear the rub strip leading edge.
Accordingly, there is a need for an improved blade positioning and support system.